Section 1

• The phylum Platyhelminthes is made up of flatworms, the simplest animals that have bilateral symmetry. Flatworms have three germ layers and are acoelomates.

• Most flatworms have a gut with a single opening, called a gastrovascular cavity. These cephalized animals also have excretory, nervous, and reproductive systems.

• The class Turbellaria consists mostly of nonparasitic flatworms, including the freshwater planarian. Planarians are sexually reproducing hermaphrodites that can also reproduce asexually by splitting in two and regenerating the missing parts.

• The classes Trematoda and Monogenea consist of parasitic flukes. Fluke life cycles include two types of hosts: a primary host, from which the adults derive their nourishment, and an intermediate host, from which the larvae derive their nourishment.

• The class Cestoda consists of parasitic tapeworms. Tapeworms do not have a digestive system, so they absorb nutrients through their body surface. Tapeworms have a series of body sections called proglottids, each of which contains reproductive structures. Tapeworm life cycles involve primary and intermediate hosts.

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